Composite materials are those materials that result when two or more materials, each having its own (usually different) characteristics, are combined to yield useful properties for specific applications. In many applications, composite materials outperform more traditional solid materials such as wood, metal, and plastic. Therefore, great interest exists in the design of strong, lightweight structures formed using composite materials.
The advanced composite industry has commensurately shown increasing interest in cost-effective processes that yield high-quality composite parts. Among these processes is resin transfer molding (RTM). Traditionally, composite part fabrication has used very little textile technology. The manufacture of all textile product forms starts with raw fiber. Discrete fiber lengths (staple fiber) can be processed into random or semi-oriented mats (non-wovens). The raw fibers can be twisted together to form a spun yarn. Continuous filament yarns are also available. Three main drawbacks plague implementation of pre-form technology for advanced composite RTM markets: (1) meeting performance requirements for engineered structures, (2) satisfying shape requirements for complex parts, and (3) reducing manufacturing costs. Current developments of textile pre-form techniques suitable for RTM attempt to overcome these drawbacks.
Typically, simple, two-dimensional (2D) woven fabrics or unidirectional fibers are produced by a material supplier and sent to a customer who cuts out patterns and lays up the final part ply-by-ply. Recently, the industry has sought to use the potential processing capabilities and economics associated with textiles to produce near-net-shape fiber assemblies or pre-forms. If designed and implemented correctly, engineered textile pre-forms with controlled fiber architecture can potentially offer a structurally efficient and cost effective fabrication of composites having various shapes and meeting stringent performance requirements.
One method of forming desired composite structures is to create matrices of extremely strong fibers which are then locked in a hardening resin. Carbon fiber, glass fibers, aramid fiber, silicon carbide fiber, and various ceramics have all been used in such materials. The resin, often an epoxy, forms the shape of the structure and holds the fibers together upon hardening, while the fibers provide exceptional tensile strength along the axes of the fibers. Composite materials may also be designed to allow flexibility perpendicular to the axes of the fibers with greatly reduced issues of fatigue from repeated cycling.
Numerous methods can be used to create the desired fiber matrix forms for such structures. Such methods include weaving, knitting, braiding, twisting, and matting. Each of these methods has both advantages and limitations. Matting is the simplest of these methods, but has as limitations that the fibers are mostly only held together by the resin, which may lead to de-lamination, and that the number of fibers pointing in a particular direction, and hence the tensile strength in that direction, is not easily controlled. Braiding and twisting are limited to substantially linear structures. Knitting forms a substantially flat structure in which most fibers are not straight. Therefore, tensile stresses will work to straighten the fibers and a composite material having a matrix of knitted fibers as a pre-form will tend to stretch to some degree. Depending on the application, this characteristic may be desirable—but it is often undesirable. A woven material will hold together and resist stretching along fiber axes, even before the addition of the resin.
The simplest woven materials are flat, substantially 2D structures with fibers in only two directions. They are formed by interlacing two sets of yarns perpendicular to each other. In 2D weaving, the 0° yarns are called the warp and the 90° yarns are called the weft, weave, or fill. Fabrics with 0° yarns and 90° yarns are produced in at least four ways. First, the number of yarns per inch may be varied in either the warp or fill direction. Second, the weaver may use a yarn with a smaller or larger filament count, which changes the weight per unit area. Third, the weaver may adjust the number of harnesses used, ranging from two (for a plain weave) to more than twenty. Each harness contains a number of heddles, or healds, loops connected to the warp yarns which move warp yarns up and down, opening and closing the shed of the loom. Fourth, the fabric can contain a mixture of fabric types in either direction. For RTM, a series of woven fabrics can be combined to form a dry layup, which is placed in a mold and injected with resin. These fabrics can be pre-formed using either a “cut and sew” technique or thermally formed and “tacked” using a resin binder.
2D woven structures have limitations. The step of pre-forming requires extensive manual labor in the layup. 2D woven structures are not as strong or stretch-resistant along other than the 0° and 90° axes, particularly at angles farther from the fiber axes. One method to reduce this possible limitation is to add bias fibers to the weave, fibers woven to cut across the fabric at an intermediate angle, preferably at +45° and −45° to the axis of the fill fibers.
Simple woven forms are also single layered. This limits the possible strength of the material. One possible solution is to increase the fiber size. Another is to use multiple layers, or plies. An additional advantage of using multiple layers is that some layers may be oriented such that the warp and weave axes of different layers are in different directions, thereby acting like the previously discussed bias fibers. If these layers are a stack of single layers laminated together with the resin, however, then the problem of de-lamination arises. If the layers are sewn together, then many of the woven fibers may be damaged during the sewing process and the overall tensile strength may suffer. In addition, for both lamination and sewing of multiple plies, a hand layup operation usually is necessary to align the layers. Alternatively, the layers may be interwoven as part of the weaving process. Creating multiple interwoven layers of fabric, particularly with integral bias fibers, has been a difficult problem. Some exemplary methods to accomplish the production of a fabric having multiple interwoven layers with bias fibers are disclosed in U.S. Pat. No. 5,540,260 issued to Mood and titled “Multi-Axial Yard Structure and Weaving Method.”
Fabrics woven by these previously described methods are still substantially 2D structures. Such fabrics are very useful for structures, such as an “L” shaped form, which do not have any junctions at which three or more sections meet. If structures having cross-sectional shapes such as “T,” “Pi,” and truss-core are formed from a substantially 2D fabric, however, then junctions must be formed either by lamination or sewing with the same flaws previously described.
Three-dimensional (3D) weaving is capable of creating fully integrated shapes with high laminar strength. Shapes such as “T,” “Pi,” and truss-core are possible without lamination or sewing. On the other hand, relative to 2D weaving, 3D weaving is more expensive and slower.
Jacquard control is one method of forming 3D woven forms. A Jacquard-control system allows individual heddles to be raised and lowered in any combination, rather than only a preset number of combinations determined by the harnesses in the loom. FIG. 10 shows a series of individual heddles 1000, holding warp yarns 102. Each of these exemplary heddles 1000 employs a hook 1002 with a clasp 1003 to hold the yarns 102. Specific heddle 1004 is shown in a raised position forming a shed.
The usefulness of this capability to individually control the heddles is demonstrated in FIGS. 9A-9E. Traditionally, heddle selection is programmed on a punched Jacquard-card which is fed through a reading mechanism on a loop, but this may also be accomplished via other digital or analog programming techniques. FIG. 9A illustrates a simple 3D form, a “T.” A single fill fiber 900 may be woven through warp fibers 902, 904, 906, 908, 910, 912, and 914 in four steps, as shown in FIGS. 9B-9E. This is only one of the possible operations to accomplish this particular weave pattern and only one of the possible weave patterns which may be used to create a “T” form.
FIG. 9B shows the fill fiber 900 being passed from left to right through a shed formed by raising warp fibers 904 and 908, while lowering the remaining warp fibers 902, 906, 910, 912, and 914. Next, as shown in FIG. 9C, warp fiber 908 is lowered and warp fibers 902, 906, and 910 are raised, then the fill fiber 900 is passed back to the left. In FIG. 9D, warp fibers 912 and 908 are raised and fill fiber 900 again passes through the shed to the right. Finally, FIG. 9E shows warp fiber 914 being raised and warp fibers 904 and 912 being lowered as fill fiber 900 returns to the left.
This weave could be accomplished using an eight-harness system as well as a Jacquard-control system. As 3D forms become more complex, however, this alternative becomes impractical. In addition, reprogramming a Jacquard system is much simpler and less time consuming than changing, and possibly reprogramming the motion of, a set of harnesses.
To overcome the shortcomings of existing weaving technology as applied to form three dimensional structures with integrally interwoven junctions and integrally interwoven bias fibers, a new weaving loom is provided. An object of the present invention is to provide improved three dimensional woven forms for RTM composite material processing. A related object is to simplify the RTM processing procedure. Another object is to simplify the addition of integrally interwoven bias fibers in woven structures.